Apparatuses including stair-step structures and methods of forming the same

ABSTRACT

Methods for forming semiconductor structures are disclosed, including a method that involves forming sets of conductive material and insulating material, forming a first mask over the sets, forming a first number of contact regions, forming a second mask over a first region of the sets, and removing material from of the sets in a second, exposed region laterally adjacent the first region to form a second number of contact regions. Another method includes forming first and second contact regions on portions of sets of conductive materials and insulating materials, each of the second contact regions more proximal to an underlying substrate than each of the first contact regions. Apparatuses such as memory devices including laterally adjacent first and second regions each of which including contact regions of a different portion of a plurality of conductive materials and related methods of forming such devices are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/679,488, filed Apr. 6, 2015, pending and which will issue Oct. 11,2016, as U.S. Pat. No. 9,466,531, which is a continuation of U.S. patentapplication Ser. No. 14/015,696, filed Aug. 30, 2013, now U.S. Pat. No.8,999,844, issued Apr. 7, 2015, which application is a continuation ofU.S. patent application Ser. No. 13/151,892, filed Jun. 2, 2011, nowU.S. Pat. No. 8,530,350, issued Sep. 10, 2013, the disclosure of each ofwhich is hereby incorporated herein in its entirety by this reference.

TECHNICAL FIELD

Embodiments of the present disclosure relate to apparatuses such asthree-dimensional semiconductor devices and systems including the same.Embodiments of the present disclosure also relate to so-called“stair-step” structures including conductive materials in so-called“stair-step” configurations for electrical connection between, forexample, memory cells and conductive lines. Other embodiments of thepresent disclosure relate to methods for forming stair-step structuresand devices including stair-step structures.

BACKGROUND

The semiconductor industry has continually sought ways to produce memorydevices with an increased number of memory cells per memory die. Innon-volatile memory (e.g., NAND flash memory), one way to increasememory density is by using a vertical memory array, which is alsoreferred to as a three-dimensional (3-D) memory array. One type ofvertical memory array includes semiconductor pillars that extend throughopenings (e.g., holes) in layers of conductive material (also referredto as word line plates or control gate plates), with dielectricmaterials at each junction of the semiconductor pillars and theconductive materials. Thus, multiple transistors can be formed alongeach pillar. Vertical memory array structures enable a greater number oftransistors to be located in a unit of die area by building the arrayupwards (e.g., vertically) on a die, as compared to structures withtraditional planar (e.g., two-dimensional) arrangements of transistors.

Vertical memory arrays and methods of forming them are described in, forexample: U.S. Patent Application Publication No. 2007/0252201 of Kito etal., now U.S. Pat. No. 7,936,004, issued May 3, 2011; Tanaka et al.,“Bit Cost Scalable Technology with Punch and Plug Process for Ultra HighDensity Flash Memory,” Symposium on VLSI Technology Digest of TechnicalPapers, pp. 14-15 (2007); Fukuzumi et al., “Optimal Integration andCharacteristics of Vertical Array Devices for Ultra-High Density,Bit-Cost Scalable Flash Memory,” IEDM Technical Digest, pp. 449-52(2007); and Endoh et al., “Novel Ultrahigh-Density Flash Memory with aStacked-Surrounding Gate Transistor (S-SGT) Structured Cell,” IEEETransactions on Electron Devices, vol. 50, no. 4, pp. 945-951 (April,2003).

Conventional vertical memory arrays require an electrical connectionbetween the conductive materials (e.g., word line plates or controlgates) and access lines (e.g., word lines) so that memory cells in the3-D array may be uniquely selected for writing or reading functions. Onemethod of forming an electrical connection includes forming a so-called“stair-step” structure at the edge of the conductive materials. FIGS. 1Athrough 1D show one conventional method of creating a stair-stepstructure 10 in a stack of conductive materials 12. As shown in FIG. 1A,conductive materials 12 are separated by insulating materials 14 betweenthe conductive materials 12. A mask 16 (e.g., photoresist material) isformed over the topmost insulating material 14 and patterned to expose aportion of the insulating material 14 a, the exposed portion having awidth of one so-called “step” of the stair-step structure 10 to beformed. An anisotropic etch 18, such as a reactive ion etch (RIE) orother dry etch, is performed to remove the insulating material 14 a atthe portion exposed through the mask 16. The pattern in the insulatingmaterial 14 a is then transferred to the conductive material 12 a. Theexposed insulating material 14 a is removed by one dry etch process thatstops on the conductive material 12 a, and the exposed conductivematerial 12 a is then removed by another dry etch process that stops onthe insulating material 14 b. Next, the mask 16 is reduced in size byremoving a portion of the mask (also known as “trimming”), such as byisotropic etching, to expose another portion of the insulating material14 a, as shown in FIG. 1B.

The process is repeated by subjecting the structure to an anisotropicetch 18, including removing exposed portions of the two insulatingmaterials 14 a and 14 b and subsequently removing exposed portions ofthe two conductive materials 12 a and 12 b. As shown in FIG. 1C, thesuccessive reduction in size of the mask 16 and the repeated dry etchprocesses are continued until the insulating material 14 c andconductive material 12 c is exposed, the mask 16 is removed, and astair-step structure 10 remains. Word line contacts 20 are formed toextend through each respective insulating material 14 and electricallycontact each conductive material 12, as shown in FIG. 1D. The top ofeach word line contact 20, as viewed in FIG. 1D, connects to aconductive word line (not shown). While FIGS. 1A through 1D illustrateusing two anisotropic etches 18 to create three so-called “steps” of thestair-step structure 10, the acts of etching the insulating material 14,etching the conductive material 12, and trimming the mask 16 may berepeated to create more steps (and thus contact regions for word linecontacts). Current conventional methods have been used to form more thaneight contact regions (e.g., steps).

As the desired number of steps in the conventional stair-step structureincreases, the margin of error associated with each act in the processof forming the steps correspondingly decreases when using theconventional method. For example, and as explained above, each iterationof the conventional method includes trimming the mask, etching theinsulating material, and etching the conductive material. The desirednumber of steps is formed by repeating these acts as many times as thenumber of conductive materials in the stack. Each act of theconventional method has an associated etch control error because thesize of each step is designed to fall within a particular range (e.g.,tolerance) to allow enough room for a contact to be formed thereon whilekeeping the overall size of the stair-step structure small.Additionally, the relative locations of the steps are designed to fallwithin a range of locations in order to accurately form contactsthereon. As the number of iterations increases, any deviation from atarget step width or location may be compounded because errors in onematerial are transferred to an underlying material. For a high number ofsteps in the stair-step structure, the margin of error to be achievedfor the etch rate control may be less than one percent (1%). Smallmargins of error are difficult and costly to attain using conventionalmethods. Furthermore, because the mask is repeatedly trimmed, the methodmay start with a mask of high thickness, which may be difficult torepeatedly pattern and trim with the precision needed to have thenecessary control over step width. Furthermore, the large amount of maskmaterial is expensive and time-consuming to both form and remove.

Space savings in a memory device incorporating a vertical memory arraymay be accomplished by reducing the area that a stair-step structurecovers. One method of reducing this area is described in U.S. PatentApplication Publication No. 2009/0310415 to Jin et al., now U.S. Pat.No. 8,325,527, issued Dec. 4, 2012. Although some space is saved byaligning the word line contacts in the same direction as the bit lines,further improvements and reductions in cost in the manufacturing of suchstructures, as well as alternative methods of reducing the area coveredby the stair-step structures, are desired. For example, the methoddescribed in Jin et al. uses a unique mask for each etch act to form thesteps, which adds significant cost because of a high number ofphotolithographic reticles used to form the masks. Reductions in costand improvements in controllability of manufacturing stair-stepstructures are, therefore, desired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1D illustrate a conventional method of forming astair-step structure for a vertical memory array.

FIGS. 2 through 10 illustrate an embodiment of a process for forming astair-step structure according to the present disclosure.

FIGS. 11 through 14 illustrate another embodiment of a process forforming a stair-step structure according to the present disclosure.

FIGS. 15 through 20 illustrate another embodiment of a process forforming a stair-step structure according to the present disclosure.

FIGS. 21 through 24 illustrate another embodiment of a process forforming a stair-step structure according to the present disclosure.

FIG. 25 illustrates an embodiment of a stair-step structure formedlaterally adjacent another stair-step structure according to the presentdisclosure.

FIG. 26 illustrates an embodiment of a stair-step structure facinganother stair-step structure with a dielectric in a valley between thefacing stair-step structures.

DETAILED DESCRIPTION

The following description provides specific details, such as materialtypes, material thicknesses, and processing conditions in order toprovide a thorough description of embodiments of the present disclosure.However, a person of ordinary skill in the art will understand that theembodiments of the present disclosure may be practiced without employingthese specific details. Indeed, the embodiments of the presentdisclosure may be practiced in conjunction with conventional fabricationtechniques employed in the industry.

The materials described herein may be formed by any suitable techniqueincluding, but not limited to, spin coating, blanket coating, chemicalvapor deposition (CVD), ALD, plasma enhanced ALD, or physical vapordeposition (PVD), unless otherwise specified. The materials may also beformed by being grown in situ. Depending on the specific material to beformed, the technique for depositing or growing the material may beselected by a person of ordinary skill in the art. While the materialsdescribed and illustrated herein may be formed as layers, the materialsare not limited thereto and may be formed in other three-dimensionalconfigurations.

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shown,by way of illustration, specific embodiments in which the presentdisclosure may be practiced. These embodiments are described insufficient detail to enable a person of ordinary skill in the art topractice the present disclosure. However, other embodiments may beutilized, and structural, logical, and electrical changes may be madewithout departing from the scope of the disclosure. The illustrationspresented herein are not meant to be actual views of any particularsystem, device, or structure, but are merely idealized representationsthat are employed to describe the embodiments of the present disclosure.The drawings presented herein are not necessarily drawn to scale.

As used herein, the term “apparatus” includes a device, such as a memorydevice (e.g., a vertical memory device), or a system that includes sucha device.

As used herein, the term “substantially” includes to a degree that oneskilled in the art would understand the given parameter, property, orcondition is met with a small degree of variance, such as withinacceptable manufacturing tolerances.

As used herein, the term “set” includes a conductive material(s) and animmediately adjacent insulating material(s). Each conductive materialcan form a word line connection separated from additional conductivematerials by the insulating material. Each insulating material mayinsulate (e.g., electrically insulate, separate, isolate from) theconductive material in its set from the conductive material of anadjacent set. The conductive material of each set may form a conductiveconnection (e.g., a word line connection) for supplying electricalsignals to a semiconductor device. Although this disclosure and theaccompanying drawings refer to sets that each include an insulatingmaterial formed over (e.g., on a side opposite a substrate) a conductivematerial, this disclosure is not so limited. A set may include aconductive material formed over (e.g., on a side opposite the substrate)an insulating material. The term “set” is used merely for ease indescribing, illustrating, and understanding the methods and structuresdisclosed.

As used herein, any relational term, such as “first,” “second,” “over,”“under,” “on,” “underlying,” “topmost,” “next,” etc., is used forclarity and convenience in understanding the disclosure and accompanyingdrawings and does not connote or depend on any specific preference,orientation, or order, except where the context clearly indicatesotherwise.

As used herein, the terms “distal” and “proximal” describe positions ofmaterials or features in relation to a substrate upon which the materialor feature is formed. For example, the term “distal” refers to aposition relatively more distant from the substrate, and the term“proximal” refers to a position in closer relative proximity to thesubstrate.

As used herein, the terms “lateral” and “laterally” refer to a directionthat is parallel to the direction that a “step” (e.g., contact region)of the stair-step structure extends. For example, the lateral directionmay be perpendicular to a direction that access lines (e.g., word lines)extend in a vertical memory device including a stair-step structure tobe described in more detail below. The lateral direction may also beparallel to a direction that bit lines extend in a vertical memorydevice including the stair-step structure. For example, the directionindicated by arrows 140 in FIG. 7 is the lateral direction.

Non-volatile memory devices (e.g., vertical memory devices, such as athree-dimensional NAND memory devices) including a plurality of contactregions on so-called “stair-steps” are disclosed, as are methods offorming such devices. A pattern of the contact regions located along anedge of the non-volatile memory device can be formed on so-called“steps” in the non-volatile memory device. A contact may be formed oneach contact region to form connections (e.g., electrical connections)to a conductive material (e.g., word line connection or control gate).While the non-volatile memory devices described herein may make specificreference to NAND devices, the disclosure is not so limited and may beapplied to other semiconductor and memory devices. Some embodiments of astair-step structure of the present disclosure and methods of formingsuch a stair-step structure are shown in FIGS. 2 through 25 and aredescribed hereafter. Similar structures or components in the variousdrawings may retain the same or similar numbering for the convenience ofthe reader; however, the similarity in numbering does not mean that thestructures or components are necessarily identical in size, composition,configuration, or any other property. Embodiments disclosed hereininclude stair-step structures (see FIGS. 9, 10, 12, 14, 20) that includeat least two regions laterally adjacent each other, each region of theat least two regions providing access to a portion of a plurality ofconductive materials. A first region may provide access to a firstportion of the plurality of conductive materials. A second region mayprovide access to a second portion of the plurality of conductivematerials different from the first portion. Embodiments disclosed hereinalso include methods of forming stair-step structures.

An embodiment of a method of forming a stair-step structure 100 forelectrical access to a vertical device (e.g., memory array) isillustrated by way of example in FIGS. 2 through 10. Alternatingconductive materials 112 and insulating materials 114 may be formed overa substrate (not shown) by conventional methods. The substrate overwhich the conductive materials 112 and insulating materials 114 areformed may be any substantially planar material. By way of non-limitingexample, the substrate may be a semiconductor material and may includeat least portions of circuits to which transistors of a memory array maybe connected. Each conductive material 112 may be used to form aconductive connection (e.g., word line connection, control gate),although the disclosure is not so limited. Each conductive material 112may, by way of non-limiting example, be a substantially planarconductive material 112. As used herein, the term “substrate” includes asemiconductor-based material including silicon, silicon-on-insulator(SOI) or silicon-on-sapphire (SOS) technology, doped and undopedsilicon, doped and undoped semiconductors, epitaxial layers of siliconsupported by a base semiconductor foundation, and other semiconductorstructures. When reference is made to a “substrate” in the followingdescription, previous process acts may have been conducted to formregions or junctions in or over the base semiconductor structure orfoundation. In addition, the semiconductor need not be silicon-based,but may be based on silicon-germanium, silicon-on-insulator,silicon-on-sapphire, germanium, or gallium arsenide, among others.

FIG. 2 shows eighteen sets 115 of conductive material 112 and insulatingmaterial 114 for clarity and ease of understanding of the drawings andrelated description. However, any number of sets 115 may be used. Inother words, fewer or more sets 115 may be used without exceeding thescope of the present disclosure. By way of example and not limitation, atotal of thirty-six conductive materials 112 and insulating materials114 may be formed to produce the eighteen sets 115. Each of theconductive materials 112 may have the same or a different compositionand each of the insulating materials 114 may have the same or adifferent composition. Furthermore, although each conductive material112 may be referred to herein in the singular form, for convenience, theconductive material 112 of each set 115 may include one or moreconductive materials. Similarly, the insulating material 114 of each setmay include one or more insulating materials. For example, theconductive material 112 of a first set 115 may include a firstconductive material formed on a second conductive material. The firstand second conductive materials of the conductive material 112 of thefirst set 115 may be separated from the conductive material(s) 112 of anadjacent second set 115 by an insulating material(s) 114.

The alternating conductive materials 112 and insulating materials 114may include an array region 122 (e.g., a vertical memory array region)and a stair-step region 124 (i.e., a region that may include astair-step after further processing) of a vertical memory device. Theconductive material 112 may be formed from any suitable conductivematerial(s). By way of example and not limitation, the conductivematerial 112 may include one or more of polysilicon and a metal, such astungsten, nickel, titanium, platinum, aluminum, gold, tungsten nitride,tantalum nitride, titanium nitride, etc. The insulating material 114 maybe formed from any suitable insulating material(s). By way of exampleand not limitation, the insulating material 114 may include a siliconoxide (e.g., SiO₂). Each set 115 of conductive material 112 andinsulating material 114 may have a thickness that is approximately 1 μm.Each of the conductive material 112 and insulating material 114 may beformed by conventional techniques, which are not described in detailherein.

The method may include forming the alternating conductive material 112and the insulating material 114 in a different order than that shown inFIG. 2. For example, in some embodiments, each set 115 may include aconductive material 112 with an insulating material 114 formed thereover(i.e., on a side opposite the substrate, as shown in FIG. 2) while inother embodiments, each set 115 may include a conductive material 112with an insulating material 114 formed thereunder (i.e., on the sameside of the conductive material 112 as the substrate, not shown). Avertical memory device resulting from such a switched configuration mayhave little or no difference in terms of functionality or operability ascompared to a vertical memory device formed by the method shown in FIGS.2 through 9.

A first mask 116 may be formed over the topmost set 115 a of conductivematerial 112 a and insulating material 114 a. The first mask 116 may bereferred to as a stair-step mask, as it is used to form a plurality ofsteps (e.g., contact regions) in the conductive material 112 andinsulating material 114. The first mask 116 may be formed of aphotoresist material, for example. The first mask 116 may be patterned,as is known in the art, to remove material from the first mask 116 at anouter edge of the stair-step region 124. The material may be removedfrom the first mask 116 to expose a portion of a major surface of thetopmost insulating material 114 a in the stair-step region 124 that hasa width 111 of approximately a desired width of the step to be formed.By way of example, a final stair-step structure (described in moredetail below) to be formed by this method may include individual steps,each exhibiting a width 111 sufficient to provide space for a conductivecontact to be formed thereon. For example, the desired width of a stepmay be in a range of from about 100 nm to about 500 nm. Therefore, thewidth 111 may be from about 100 nm to about 500 nm. In some embodiments,the width 111 may be from about 220 nm to about 250 nm. However, theseparticular widths are described by way of example only, and notlimitation. The width 111 may be greater or less than the particularwidths described.

As used herein, the phrase “to expose” includes to uncover a majorsurface of a material. For example, the insulating material 114 a shownin FIG. 2 includes a portion of a major surface thereof that is exposed.

After the first mask 116 is patterned, the portion of the insulatingmaterial 114 a exposed through the first mask 116 may be removed by, forexample, an anisotropic etch 118. By way of example, the anisotropicetch 118 may include a first dry etch act that removes the exposedportion of the insulating material 114 a and exposes the conductivematerial 112 a, followed by a second dry etch act that removes a portionof the conductive material 112 a that was exposed by the first dry etchact. The second dry etch act of the anisotropic etch 118 may expose theinsulating material 114 b. One instance of the first dry etch act andthe second dry etch act may be referred to herein as a cycle of theanisotropic etch 118. Since the first dry etch act and the second dryetch act remove the portion of the insulating material 114 a and theportion of the conductive material 112 a, the first dry etch act and thesecond dry etch act may remove a portion of the first set 115 a.Although the method described herein refers to an anisotropic etch 118,the disclosure is not so limited. For example, an isotropic etch may beused in place of the anisotropic etch 118.

The first dry etch act of each cycle of the anisotropic etch 118 mayselectively remove the insulating material 114. In other words, thefirst dry etch act may remove exposed portions of the insulatingmaterial 114 and stop removing material once the conductive material 112is at least partially exposed. The first dry etch act of the anisotropicetch 118 may be performed longer than necessary to ensure thatsubstantially all of the exposed insulating material 114 is removed andthe conductive material 112 immediately thereunder is substantiallycompletely exposed. Similarly, the second dry etch act of each cycle ofthe anisotropic etch 118 may selectively remove the exposed portions ofthe conductive material 112 and stop removing material once theinsulating material 114 immediately thereunder is exposed. The seconddry etch act of the anisotropic etch 118 may be performed longer thannecessary to ensure that substantially all of the conductive material112 is removed and the insulating material 114 immediately thereunder issubstantially completely exposed. The dry etch acts will be apparent toa person having ordinary skill in the art and, thus, are not describedin detail herein.

A portion of the first mask 116 may then be removed to expose anotherportion of the first insulating material 114 a, resulting in thestructure shown in FIG. 3. The portion of the first mask 116 may beremoved by, for example, an isotropic etch that is selective to thematerial of the first mask 116 and that does not substantially removethe material of the insulating material 114 or the conductive material112. Material from the first mask 116 may be removed to the extent thatthe portion of the first insulating material 114 a that is exposedexhibits a width that is approximately a desired width of a step, asdescribed above.

Another anisotropic etch 118 may be used to remove exposed portions ofthe insulating materials 114 a and 114 b and subsequently exposedportions of the conductive materials 112 a and 112 b thereunder. Inother words, exposed portions of the set 115 a and the set 115 b may beremoved by one cycle of the anisotropic etch 118. A portion of the firstmask 116 may be removed again to expose yet another portion of theinsulating material 114 a, resulting in the structure shown in FIG. 4.

The acts of removing a portion of the first mask 116, removing theexposed insulating material 114, and removing the exposed conductivematerial 112 may be repeated a plurality of times to expose insulatingmaterial 114 j and form steps in sets 115 a through 115 i, which coversone-half of the total number of sets 115, as shown in FIG. 5. In otherwords and by way of example, where the total number of sets 115 iseighteen, the ninth insulating material 114 i (when countingsequentially starting with insulating material 114 a) may have anexposed step formed therein, while the tenth insulating material 114 jthereunder may not have a step formed therein. The tenth insulatingmaterial 114 j may have a portion thereof exposed. The remainder of thefirst mask 116 may then be removed by conventional techniques, which arenot described in detail herein. By way of example, the first mask 116may be substantially removed from the surface of the first insulatingmaterial 114 a with a dry or wet etch act.

As can be seen in FIG. 6, an intermediate stair-step structure 150 maybe formed in which half the number of sets 115 of the total number ofsets 115 includes steps formed therein. By way of example and as shownin FIG. 6, for a structure with eighteen sets 115, the method asdescribed may be used to form an intermediate stair-step structure 150with nine sets 115 having steps formed therein, while the remaining ninesets 115 do not include steps formed therein.

Referring now to FIG. 7, a second mask 126 (also referred to as a “chopmask 126”) may be formed over the insulating materials 114 a through 114i and patterned to cover the array region 122 (not shown in FIG. 7) anda first region 170 of the stair-step region 124, while leaving a secondregion 180 of the stair-step region 124 exposed. The second region 180may be a region that is laterally (i.e., in the direction of the arrows140 in FIG. 7) adjacent the first region 170. By way of example, thesecond region 180 may have a length 182 determined by the configurationand size of word lines that will eventually connect to the stair-stepstructure 100 by way of word line contacts. In an embodiment where eachset 115 has a thickness of about 1 μm, the length 182 exposed throughthe mask 126 may be about 3 μm.

The second region 180 may then be subjected to an anisotropic etch 128,including enough cycles to remove material from a number of sets 115 toexpose the sets 115 of the bottom half of the stack, as shown in FIG. 8.By way of example, the anisotropic etch 128 may include: a first dryetch act that selectively removes exposed portions of the insulatingmaterial 114; a second dry etch act that removes portions of theconductive material 112 that are exposed by the first dry etch act; athird dry etch act that removes portions of the insulating material 114that are exposed by the second dry etch act; and so forth until thedesired amount of material is removed. By way of non-limiting exampleand as shown in FIGS. 7 and 8, the anisotropic etch 128 may be repeatedin situ nine times to remove portions of nine exposed sets 115.

As can be seen in FIG. 9, the second mask 126 may be removed, providingaccess (i.e., exposure) to each set 115 and forming a stair-stepstructure 100. The stair-step structure 100 may include a first region170 providing exposure to a first half of the sets 115 and a secondregion 180 laterally adjacent the first region 170 providing exposure toa second half of the sets 115. Thus, each set 115 may be accessible toform a conductive contact thereon electrically connected to eachconductive material 112, respectively. In other words, an exposedportion of each set 115 may be referred to as a “contact region.” Eachcontact region may be offset from other contact regions. As used herein,the term “offset” includes located at a different distance from asubstrate. For example, a first contact region offset from a secondcontact region may refer to the contact region of a first set and thecontact region of a second set different from the first set, as shown inFIG. 9. The contact regions of the stair-step structure 100 may extendat an angle 190 from the substrate, as will be explained in more detailbelow.

Referring now to FIG. 10, conductive contacts 120 may be formed toprovide electrical contact to each conductive material 112. By way ofexample, a second insulating material (not shown), such as one or moreof a silicon oxide, borophosphosilicate glass (BPSG), and a spin-ondielectric (SOD), may be formed over the stair-step region 124. Thesecond insulating material may optionally be planarized by, for example,an abrasive planarization process, such as a chemical-mechanicalpolishing (CMP) process or a mechanical polishing process. Openings maybe formed through the second insulating material and through theinsulating material 114 (where the insulating material 114 covers thetop of each contact area of the stair-step structure 100, as shown inFIG. 10) to expose the underlying conductive materials 112. The openingsmay then be filled with a conductive material (e.g., polysilicon,tungsten, titanium, aluminum, etc.) to form the conductive contacts 120that electrically connect to the conductive materials 112, respectively,as shown in FIG. 10. By way of example, the conductive contacts 120 maybe word line contacts. Conductive word lines may then be formed (notshown) and connected to each word line contact 120. The length 182 (seeFIG. 7) of the second region 180 of the stair-step region 124 may bechosen to provide sufficient space for the word lines and word linecontacts 120 to be formed and electrically isolated from one another.

Alternatively, the conductive contacts 120 may be formed to have aconfiguration different than that shown in FIG. 10. By way of example,the conductive contacts 120 may be formed to extend from each conductivematerial 112 through the stair-step structure 100 to the substraterather than or in addition to away from the substrate. For example, U.S.patent application Ser. No. 13/151,945, filed Jun. 2, 2011, now U.S.Pat. No. 8,765,598, issued Jul. 1, 2014, and assigned to the Assignee ofthe present application, describes contacts extending toward thesubstrate through a stair-step structure and methods of forming suchcontacts. In other words, this disclosure is not limited to formingcontacts extending from the materials of the stair-step structure 100 ina direction away from the substrate, as shown in FIG. 10.

The method illustrated in FIGS. 2 through 10 results in a stair-stepstructure 100 having two distinct regions 170 and 180. The first region170 of the stair-step structure 100 includes contact areas allowingelectrical access to a first portion of the sets 115 of insulatingmaterials 114 and conductive materials 112. The second region 180 of thestair-step structure 100 is laterally adjacent the first region 170 andincludes contact areas allowing electrical access to a second portion ofthe sets 115. The first region 170 may include contact areas of one-halfof the sets 115 and the second region 180 may include contact areas ofthe other half of the sets 115.

By utilizing the embodiment illustrated in FIGS. 2 through 10 anddescribed in the accompanying text, a stair-step structure 100 may beformed to provide access to each set 115. The stair-step structure 100extends only half as far out from the array region 122 (see FIG. 5) ascompared to prior art stair-step structures having the same number ofsets of insulating material 114 and conductive material 112, savingspace and reducing the size of a memory device including the stair-stepstructure 100. Additionally, the stair-step structure 100 may be formedproximate another stair-step structure (not shown) of another verticaldevice. A gap between the two stair-step structures may be smaller thana conventional gap, which may be cheaper and easier to fill withmaterial (e.g., the second insulative material described above) insubsequent steps. Because the resulting fill material would span ashorter distance, planarization of the resulting fill material may beaccomplished more easily and controllably. The method described in FIGS.2 through 10 also may have the advantage of forming a large number ofsteps while greatly reducing a manufacturing margin of error. By way ofexample, the act of removing portions of the first mask 116 may be doneonly eight times to form the eighteen steps of the stair-step structure100. In contrast, conventional methods may require trimming back of amask seventeen times to form a stair-step structure with eighteen steps.Any error in trimming the mask may, therefore, be multiplied up to eighttimes rather than seventeen times, as would be done with conventionalmethods. In other words, the method described in this disclosure mayprovide a lower margin of error to form a stair-step structure 100 ascompared to a stair-step structure formed by conventional methods.Furthermore, the method described in this disclosure may include formingtwo masks (first mask 116 and second mask 126) to achieve a manageablemargin of error and space savings, rather than using a larger number ofmasks as would be done with conventional methods. Since each mask mayrequire an additional photolithographic reticle to be formed, the largenumber of masks adds significant expense to the manufacturing process.Thus, reducing the number of masks used to attain a better margin oferror provides cost savings.

Another embodiment of a method for forming a stair-step structure isillustrated in FIGS. 11 through 14. The method may begin in a similarfashion to that illustrated in FIGS. 2 through 6 to form an intermediatestair-step structure 250. However, the intermediate stair-step structure250 of this embodiment is different than the intermediate stair-stepstructure 150 shown in FIG. 6 because the intermediate stair-stepstructure 250 includes exposure of, and steps formed in, one-fourth ofsets 215 of insulating material 214 and conductive material 212, ratherthan one-half of the sets 115 as in the intermediate stair-stepstructure 150 shown in FIG. 6. By way of non-limiting example and forease of illustration, the structure shown in FIG. 11 includes sixteentotal sets 215. However, any desired number of sets 215 may be used.Steps may be formed into the four topmost sets 215 (i.e., one-fourth ofthe sixteen total sets 215) essentially as described above withreference to FIGS. 2 through 6 to expose the insulating material 214 ofeach of the four topmost sets 215. A second mask 236 may be formed overthe insulating materials 214 a through 214 d and patterned to expose thesets in a second region 260 laterally adjacent a first region 240 of theintermediate stair-step structure 250.

An anisotropic etch 238 may be performed including four cycles ofremoving insulating material 214 and conductive material 212 to exposeand form steps in the next four sets 215 (215 e through 215 h) in theexposed second region 260, as illustrated in FIGS. 11 and 12. After theanisotropic etch 238 is performed, the second mask 236 may be removed.Thus, the four topmost sets 215 a through 215 d may be exposed and havesteps formed therein in a first region 240 and the next four sets 215 ethrough 215 h may be exposed and have steps formed therein in a secondregion 260 laterally adjacent the first region 240. The bottom half ofthe sets 215 (i.e., the eight sets 215 below the eighth set 215 h inFIG. 12) may not have steps formed therein. The conductive material 214i of the ninth set 215 i may have a portion thereof exposed after theformation of a step in the eighth set 215 h.

Referring now to FIG. 13, a third mask 246 may be formed over exposedinsulating material 214 a through 214 d of a first portion 241 of thefirst region 240 (FIG. 12) and over exposed insulating material 214 ethrough 214 h of a first portion 261 of the second region 260 (FIG. 12)and patterned to expose a second portion 243 of the first region 240 anda second portion 263 of the second region 260. By way of non-limitingexample, about one-half of each of the first region 240 and of thesecond region 260 may be covered by the third mask 246 while theremaining about one-half may be exposed.

Another anisotropic etch 248 may be conducted to remove the exposedmaterial. By way of example, the anisotropic etch 248 may include eightcycles of removing insulating material 214 and conductive material 212to remove material from eight sets 215 in each of the exposed portions243 and 263. As can be seen in FIG. 14, a resulting stair-step structure200 may include four distinct portions 243, 241, 261, and 263 laterallyadjacent one another, each providing exposure to a distinct one-fourthof the total number of sets 215. Conductive contacts (not shown) maythen be formed to provide electrical contact with each conductivematerial 212 essentially as described above with reference to FIG. 10.

It is noted that variations of the embodiment shown in FIGS. 11 through14 may exist that do not exceed the scope of the present disclosure. Forexample, the anisotropic etch 238 shown in FIG. 11 may remove eight sets215 rather than four sets 215 and the anisotropic etch 248 shown in FIG.13 may remove four sets 215 rather than eight sets 215. Although thenumber of cycles of each anisotropic etch is changed, a resultingstructure may still include four distinct regions that each providesaccess to one-fourth of the sets 215. It is also noted that the specificnumber of sets 215 shown in FIGS. 11 through 14 is sixteen forconvenience; however, the disclosure is not so limited. A stair-stepstructure with any desired number of sets 215 of insulating material 214and conductive material 212 may be formed by the methods disclosed.

The embodiment of the method of the disclosure shown in FIGS. 11 through14 may provide some advantages over the embodiment illustrated in FIGS.2 through 10. For example, an even more attainable margin of error maybe realized because the first mask (i.e., the stair-step mask) may betrimmed fewer times to gain the same number of contact regions (e.g.,steps). In addition, the distance that a stair-step region 224 of themethod shown in FIGS. 11 through 14 extends from an array region (e.g.,a vertical memory array region) is less than the distance that thestair-step region 124 of the method shown in FIGS. 2 through 10; thusadditional space savings may be realized. However, an additionalphotolithographic reticle may be used to form the third mask 246 shownin FIG. 13 that is omitted in the method shown in FIGS. 2 through 10.Therefore, there may be some additional cost in manufacturing thestair-step structure 200 formed by the method shown in FIGS. 11 through14. Thus, a method of forming a stair-step structure of the presentdisclosure may be modified and tailored to the costs and benefitsinvolved in a given situation.

Another embodiment of a method of forming a stair-step structure forelectrical access to a vertical device (e.g., memory array) isillustrated by way of example in FIGS. 15 through 20. Referring to FIG.15, a number of sets 315 of alternating conductive material 312 andinsulating material 314 may be formed. By way of example and forclarity, eighteen sets 315 are shown, although the disclosure is not solimited. A first mask 316 (also referred to as a “stair-step mask 316”)may be formed over insulating material 314 a to cover both an arrayregion 322 (e.g., a vertical memory array region) and a stair-stepregion 324. The first mask 316 may be patterned to expose a portion ofthe insulating material 314 a with a width about the same width as adesired step of a stair-step structure to be formed, essentially asdescribed above with reference to FIG. 2.

An anisotropic etch 318 may be performed to remove a portion of the twouppermost sets 315 a and 315 b. In other words, the anisotropic etch 318may remove the exposed portion of the insulating material 314 a, theunderlying portion of the conductive material 312 a, the underlyingportion of the next insulating material 314 b, and the underlyingportion of the next conductive material 312 b. In other words, twocycles of the anisotropic etch 318 may be performed to remove portionsof two sets 315 of conductive material 312 and insulating material 314,rather than one cycle of anisotropic etching 318 through one set 315. Aportion of the first mask 316 may then be removed to expose anotherportion of the insulating material 314 a, essentially as described abovewith reference to FIG. 2.

Referring now to FIG. 16, a portion having a width of about one desiredstep width of the insulating material 314 a may be exposed as well as asimilar portion of the third insulating material 314 c. Anotheranisotropic etch 318 may be performed, again etching through two sets315 of conductive material 312 and insulating material 314. Anotherportion of the first mask 316 may be removed through an isotropic etch,resulting in the structure shown in FIG. 17. The sequential anisotropicetching 318 and removal of portions of the first mask 316 may continueuntil steps have been formed in a desired fraction, such as one-half, ofthe sets 315 and an intermediate stair-step structure 350 is formed, asshown in FIG. 18. In other words, every other set 315 (when proceedingfrom a substrate towards a topmost set 315 a) may have at least aportion of its insulating material 314 exposed, the exposed portionhaving a width sufficient to form a conductive contact thereon ortherethrough.

Referring now to FIG. 19, a second mask 326 (also referred to as a “chopmask 326”) may be formed over the exposed insulating materials 314 andpatterned to cover a first region 370 of the stair-step region 324 andto expose a second region 380 laterally adjacent the first region 370.One cycle of another anisotropic etch 328 may be performed on theexposed second region 380 to remove an exposed portion of one set 315from each step. In other words, the anisotropic etch 328 may removeexposed insulating material 314 from each of the exposed sets 315 andthen may remove underlying conductive material 312 from each of theexposed sets 315. In this manner, the sets 315 that were not exposed inthe intermediate stair-step structure 350 shown in FIG. 18 may beexposed by performing one cycle of the anisotropic etch 328.

The second mask 326 may be removed, resulting in the stair-stepstructure 300 shown in FIG. 20. The stair-step structure 300 may includea first region 370 providing exposure of (i.e., access to) one-half ofthe sets 315. The first region 370 may provide exposure of every otherset 315. The stair-step structure 300 may also include a second region380 laterally adjacent the first region 370 providing exposure of theother half of the sets 315.

An angle 390 of a stair-step structure 300 formed by the methodillustrated in FIGS. 15 through 20 may be less than an angle 190 of astair-step structure 100 formed by the method illustrated in FIGS. 2through 10. A device including a stair-step structure 300 may alsoinclude another stair-step structure facing the first. For example, FIG.26 illustrates the stair-step structure 350 of FIG. 18 facing anotherstair-step structure 350. A steeper stair-step structure (i.e., asmaller angle) may result in a valley between neighboring stair-stepstructures with less width than a valley between stair-step structuresthat are less steep (i.e., having a larger angle). A valley 304 havingsuch a smaller width may be easier, and therefore cheaper, to fill withdielectric 302 or other desired materials. The valley 304 with a smallerwidth filled with the dielectric 302 or other desired material may alsobe easier to planarize, such as before forming contacts providingelectrical connection to the conductive materials of the stair-stepstructure.

Another embodiment of a method for forming a stair-step structure, suchas the stair-step structure 300 shown in FIG. 20, is illustrated inFIGS. 21 through 24. As shown in FIG. 21, alternating conductivematerials 412 and insulating materials 414 may be formed over asubstrate (not shown) to form a plurality of sets 415, each set 415including one or more conductive material(s) 412 and one or moreinsulating material(s) 414. By way of example and not limitation,eighteen sets 415 are shown for clarity, although the disclosure is notso limited. A first mask 426 (also referred to as a “chop mask 426”) maybe formed over the insulating material 414 a and patterned to expose theinsulating material 414 a in a second region 480 laterally adjacent afirst region 470. The first mask 426 may also be formed to cover anarray region (not shown). Material from the set 415 a including theinsulating material 414 a and the conductive material 412 a may beremoved in the second region 480 by a cycle of an anisotropic etch 428.

As shown in FIG. 22, the set 415 a may remain in the first region 470and be removed in the second region 480, exposing the insulatingmaterial 414 b in the second region 480. The first mask 426 may beremoved from the first region 470 to expose the insulating material 414a in the first region 470. Referring now to FIG. 23, a second mask 436(also referred to as a “stair-step mask 436”) may be formed over boththe first region 470 and the second region 480 and patterned to exposeapproximately one stair-width of the insulating materials 414 a and 414b. Material from the two sets 415 a and 415 b may be removed by ananisotropic etch 438 in the first region 470 and material from the twosets 415 b and 415 c may be removed by the anisotropic etch 438 in thesecond region 480. In other words, the anisotropic etch 438 may includetwo cycles of etching through insulating material 414 and conductivematerial 412. A portion of the second mask 436 may then be removed toexpose a portion of the two sets 415 a and 415 b having a width ofapproximately one stair-width, as illustrated in FIG. 24.

In the first region 470, portions of the insulating material 414 a andportions of the insulating material 414 c may be exposed. In the secondregion 480 laterally adjacent the first region 470, portions of thesecond insulating material 414 b and the fourth insulating material 414d may be exposed. Another anisotropic etch 438 may be performed toremove exposed portions of the sets 415 a, 415 b, 415 c, and 415 d,again removing two sets 415 in each exposed portion.

The acts of removing a portion of the second mask 436 andanisotropically etching 438 through exposed portions of two sets 415 maybe repeated a plurality of times until a portion of each set 415 isexposed. The second mask 436 may then be removed. The embodimentillustrated and described with reference to FIGS. 21 through 24 mayresult in a stair-step structure 300 substantially identical inconfiguration to the stair-step structure 300 shown in FIG. 20.

In some embodiments, multiple stair-step structures 100, 200, or 300 maybe formed simultaneously following the methods described herein, as willbe appreciated by one skilled in the art. By way of example and asillustrated in FIG. 25, a first stair-step structure 100 a may be formedas described in more detail hereinabove to include a first region 170 aand a second region 180 a laterally adjacent the first region 170 a. Atthe same time and by following the same methods, a second stair-stepstructure 100 b may be formed laterally adjacent the first stair-stepstructure 100 a. For example, a first region 170 b of the secondstair-step structure 100 b may be formed laterally adjacent the secondregion 180 a of the first stair-step structure 100 a. The firststair-step structure 100 a may be electrically insulated from the secondstair-step structure 100 b by way of an insulating material (not shown),which could be a void, disposed between the first and second stair-stepstructures 100 a and 100 b.

In some embodiments, a vertical memory device may include a stair-stepstructure, such as the stair-step structures 100, 200, or 300. Eachconductive material 112, 212, 312, or 412 of the stair-step structuremay function as a conductive connection (e.g., a word line connection).Each conductive material may provide electrical access to transistors(not shown) in a particular plane in a vertical memory array. Thus, anyof the stair-step structures 100, 200, or 300 disclosed herein may beused in a vertical memory device, or other vertical device.

Several embodiments of stair-step structures and methods for formingstair-step structures have been shown and described. These embodimentsmay have advantages as compared to conventional structures and methods.For example, a margin of error in forming the plurality of stair-stepsmay be kept to a more reasonable and attainable level by utilizing themethods of the present disclosure as compared to conventional methods.The improvement in the margin of error may be achieved without theexpense of utilizing large numbers of reticles that may be required forforming a plurality of masks. In addition, a stair-step structure of thepresent disclosure may cover less area than some conventional stair-stepstructures. The smaller area covered by the stair-step structure mayallow a device including the stair-step structure to be produced moreefficiently and cost-effectively. Furthermore, a space between adjacentstair-step structures may be reduced in size and therefore easier and/orcheaper to fill and planarize with material in subsequent manufacturingprocesses.

CONCLUSION

In one embodiment, a method of forming a semiconductor structure isdescribed, including forming a plurality of sets of conductive materialand insulating material, forming a first mask over a topmost set of theplurality of sets, removing a portion of the first mask to expose aportion of a major surface of the topmost set, removing the exposedportion of the topmost set, removing another portion of the first maskto expose another portion of the topmost set, and repeating the removinga portion of the first mask and removing the exposed portion of thetopmost set until a first number of contact regions are formed. Themethod also includes forming a second mask over a first region of theplurality of sets and removing material from of the plurality of sets ina second, exposed region of the plurality of sets laterally adjacent thefirst region to form a second number of contact regions.

In a further embodiment, a method of forming a stair-step structure isdescribed, including forming first contact regions on portions of setsof conductive materials and insulating materials, forming a mask over aregion of the sets, and removing a portion of the sets not covered bythe mask to form second contact regions. Each of the second contactregions may be more proximal to an underlying substrate than each of thefirst contact regions.

In an additional embodiment, a method of forming a vertical memorydevice is described, including forming a plurality of alternating wordline connections and insulating materials and forming contact regions onportions of the word line connections. The method also includes forminga mask over a region of the alternating word line connections andinsulating materials, removing a portion of the word line connectionsand insulating materials not covered by the mask, and forming contactsin connection with each contact region.

In another embodiment, an apparatus is described that includes aplurality of sets of conductive material and insulating material. Afirst region of the plurality of sets includes contact regions of afirst portion of the plurality of sets and a second region of theplurality of sets includes contact regions of a second portion. Thefirst region and the second region of the plurality of sets arelaterally adjacent one another. The contact regions of the first regionare offset from the contact regions of the second region.

In an additional embodiment, an apparatus is described, including avertical memory array region including a plurality of conductivematerials and a stair-step region also including the plurality ofconductive materials. The stair-step region includes a first region anda second region laterally adjacent the first region. The first regionincludes contact regions of a first portion of the plurality ofconductive materials and the second region includes contact regions of asecond portion of the plurality of conductive materials different thanthe first portion. Each contact region of the first region is offsetfrom each contact region of the second region.

In yet another embodiment, a method of forming a semiconductor structureis described including forming a plurality of sets of conductivematerial and insulating material, removing a first portion of a topmostset of the plurality, forming a mask over a first region of a secondportion of the topmost set and leaving a second region of the secondportion exposed, and removing the exposed second portion of the topmostset.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, the invention is not intended to be limited to the particularforms disclosed. Rather, the invention is to cover all modifications,combinations, equivalents, and alternatives falling within the scope ofthe invention as defined by the following appended claims and theirlegal equivalents. For example, while embodiments of the presentdisclosure have been described as relating to vertical memory devices,other devices may include methods and structures similar to thosedescribed without exceeding the scope of the present disclosure. Forexample, a stair-step structure formed by the methods disclosed hereinmay be used to form contact regions for vertically stacked wiringbusses, capacitors, or any other set of laminated conductors. In otherwords, any stair-step structure may be formed by the methods disclosedherein and be configured identically or similarly to the structuresdisclosed herein.

What is claimed is:
 1. A semiconductor structure, comprising: a firststair-step structure comprising conductive materials and insulatingmaterials, the first stair-step structure including a first set ofcontact regions, each contact region of the first set of contact regionsvertically offset from other contact regions of the first set of contactregions; a second stair-step structure isolated from the firststair-step structure and comprising the conductive materials and theinsulating materials, the second stair-step structure including a secondset of contact regions, each contact region of the second set of contactregions vertically offset from other contact regions of the second setof contact regions; and a valley between the first stair-step structureand the second stair-step structure.
 2. The semiconductor structure ofclaim 1, wherein the first stair-step structure and the secondstair-step structure face each other to define the valley therebetween.3. The semiconductor structure of claim 1, wherein the valley is filledwith another insulating material.
 4. The semiconductor structure ofclaim 1, wherein each of the first stair-step structure and the secondstair-step structure is proximate an array region of the semiconductorstructure.
 5. The semiconductor structure of claim 1, further comprisinga conductive contact electrically connected to each of the conductivematerials of the first stair-step structure and each of the conductivematerials of the second stair-step structure.
 6. The semiconductorstructure of claim 1, further comprising conductive contacts extendingthrough an insulating material in the valley to the contact regions ofthe first stair-step structure and to the contact regions of the secondstair-step structure.
 7. The semiconductor structure of claim 1, whereinthe first set of contact regions comprises: a first region comprisingfirst contact regions to a first group of conductive materials; and asecond region spaced from the first region and comprising second contactregions to a second group of conductive materials, the second contactregions vertically offset from the first contact regions.
 8. Thesemiconductor structure of claim 7, wherein the first set of contactregions comprises: a third region comprising third contact regions to athird group of conductive materials; and a fourth region comprisingfourth contact regions to a fourth group of conductive materials.
 9. Thesemiconductor structure of claim 1, wherein the second stair-stepstructure comprises a mirror image of the first stair-step structure.10. A semiconductor structure including a stair-step structure, thestair-step structure comprising: a plurality of sets of conductivematerial and insulating material; a first region comprising a first setof contact regions comprising a first portion of sequential sets of theplurality of sets, the first set of contact regions extending in a firstdirection from a top first contact region to a bottom first contactregion; and a second region spaced from the first region, the secondregion comprising a second set of contact regions comprising a secondportion of sequential sets of the plurality of sets, the second set ofcontact regions extending in the first direction, wherein a top contactregion of the second set of contact regions is higher than all of thecontact regions of the first set of contact regions.
 11. Thesemiconductor structure of claim 10, wherein the second region is spacedfrom the first region by another insulating material.
 12. Thesemiconductor structure of claim 10, further comprising a third regionspaced from at least one of the first region or the second region, thethird region comprising a third set of contact regions comprising athird portion of sequential sets of the plurality of sets, the contactregions of the third set of contact regions vertically offset from thefirst contact regions and the second contact regions.
 13. Thesemiconductor structure of claim 10, further comprising a fourth regioncomprising a fourth set of contact regions comprising a fourth portionof sequential sets of the plurality of sets, the contact regions of thefourth set of contact regions vertically offset from the first contactregions, the second contact regions, and the third contact regions. 14.The semiconductor structure of claim 10, further comprising a respectiveconductive contact in electrical communication with each contact regionof the first set of contact regions and of the second set of contactregions.
 15. The semiconductor structure of claim 10, wherein theconductive material comprises a metal.
 16. A method of forming asemiconductor structure including at least one stair-step structure, themethod comprising: forming a plurality of sets of conductive materialand insulating material; forming a first set of contact regionscomprising a first portion of sequential sets of the plurality of sets,the first set of contact regions extending in a first direction from atop first contact region to a bottom first contact region; forming asecond set of contact regions spaced from the first set of contactregions and comprising a second portion of sequential sets of theplurality of sets, the second set of contact regions extending in thefirst direction, wherein a top second contact is higher than all of thecontact regions of the first set of contact regions.
 17. The method ofclaim 16, wherein forming a first set of contact regions comprises:forming a mask over a topmost set of conductive material and insulatingmaterial of the plurality of sets of conductive material and insulatingmaterial; removing a portion of the mask to expose a portion of thetopmost insulating material; and removing at least a portion of theconductive material of the topmost set of conductive material andinsulating material.
 18. The method of claim 16, wherein forming a firstset of contact regions comprises: forming a mask over a first region ofthe plurality of sets of conductive material and insulating material;and removing some of the sets of the plurality of sets of conductivematerial and insulating material while the mask remains over the firstregion.
 19. The method of claim 16, wherein forming a second set ofcontact regions spaced from the first set of contact regions comprisesdisposing another insulating material between the first set of contactregions and the second set of contact regions.
 20. The method of claim16, wherein forming a second set of contact regions comprises forming astair-step structure comprising the second portion of sequential sets ofthe plurality of sets facing a stair-step structure of the first region.21. The method of claim 16, further comprising selecting the conductivematerial to comprise a metal.